✨XMAS NUT MERGED: NUT-20 – Signature on Mint Quote

This optional NUT specifies how to lock a mint quote to a user-defined public key and allow minting only with a valid signature. This improves the security of the Cashu protocol for bolt11, will improve bolt12 mint quotes that can be paid multiple times, and allows building trust-minimzed services that can mint tokens for its users without having access to the ecash (great for services like LNURL).

Thank you @thesimplekid for leading the work on this!